Luma-Based Chroma Intra-Prediction for Video Coding

ABSTRACT

A method for luma-based chroma intra-prediction in a video encoder or a video decoder is provided that includes down sampling a first reconstructed luma block of a largest coding unit (LCU), computing parameters α and β of a linear model using immediate top neighboring reconstructed luma samples and left neighboring reconstructed luma samples of the first reconstructed luma block and reconstructed neighboring chroma samples of a chroma block corresponding to the first reconstructed luma block, wherein the linear model is Pred C [x,y]=α·Rec L ′[x,y]+β, wherein x and y are sample coordinates, Pred C  is predicted chroma samples, and Rec L ′ is samples of the down sampled first reconstructed luma block, and wherein the immediate top neighboring reconstructed luma samples are the only top neighboring reconstructed luma samples used, and computing samples of a first predicted chroma block from corresponding samples of the down sampled first reconstructed luma block using the linear model and the parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/501,031, filed Jun. 24, 2011, which is incorporated hereinby reference in its entirety. This application is related to co-pendingU.S. patent application Ser. No. 13/470,186, filed May 11, 2012, whichis incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to luma-basedchroma intra-prediction in video coding.

2. Description of the Related Art

Video compression, i.e., video coding, is an essential enabler fordigital video products as it enables the storage and transmission ofdigital video. In general, video compression techniques applyprediction, transformation, quantization, and entropy coding tosequential blocks of pixels in a video sequence to compress, i.e.,encode, the video sequence. Video decompression techniques generallyperform the inverse of these operations in reverse order to decompress,i.e., decode, a compressed video sequence.

The Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T WP3/16and ISO/IEC JTC 1/SC 29/WG 11 is currently developing thenext-generation video coding standard referred to as High EfficiencyVideo Coding (HEVC). HEVC is expected to provide around 50% improvementin coding efficiency over the current standard, H.264/AVC, as well aslarger resolutions and higher frame rates. To address theserequirements, HEVC utilizes larger block sizes than H.264/AVC. In HEVC,the largest coding unit (LCU) can be up to 64×64 in size, while inH.264/AVC, the macroblock size is fixed at 16×16.

Several coding efficiency enhancement tools are proposed in HEVC toreduce coding overhead. One such coding tool is luma-based chromaintra-prediction, also referred to as linear method mode (LM mode). Ingeneral, in luma-based chroma intra-prediction, chroma values in a blockare predicted from down sampled reconstructed luma samples in the sameblock. More detailed descriptions of this coding tool for HEVC may befound, for example, in J. Kim, et al., “New Intra Chroma PredictionUsing Inter-Channel Correlation,” JCTVC-B021, Joint Collaborative Teamon Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11,Geneva, CH, Jul. 21-28, 2010, J. Chen and V. Seregin, “Chroma IntraPrediction by Reconstructed Luma Samples,” JCTVC-C206, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG11, Guangzhou, Conn., Oct. 7-15, 2010, J. Chen, etal., “CE6.a: Chroma Intra Prediction by Reconstructed Luma Samples,”JCTVC-D350, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-TSG16 WP3 and ISO/IEC JTC1/SC29/WG11, Daegu, KR, Jan. 20-28, 2011, and J.Chen, et al., “CE6.a.4: Chroma Intra Prediction by Reconstructed LumaSamples,” JCTVC-E266, Joint Collaborative Team on Video Coding (JCT-VC)of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, Geneva, CH, Mar. 16-23,2011.

SUMMARY

Embodiments of the present invention relate to methods, apparatus, andcomputer readable media for luma-based chroma intra-prediction in videocoding. In one aspect, a method for luma-based chroma intra-predictionin a video encoder or a video decoder is provided that includes downsampling a first reconstructed luma block of a largest coding unit(LCU), computing parameters α and β of a linear model using immediatetop neighboring reconstructed luma samples and left neighboringreconstructed luma samples of the first reconstructed luma block andreconstructed neighboring chroma samples of a chroma block correspondingto the first reconstructed luma block, wherein the linear model isPred_(C)[x,y]=α·Rec_(L)′[x,y]+β, wherein x and y are sample coordinates,Pred_(C) is predicted chroma samples, and Rec_(L)′ is samples of thedown sampled first reconstructed luma block, and wherein the immediatetop neighboring reconstructed luma samples are the only top neighboringreconstructed luma samples used, and computing samples of a firstpredicted chroma block from corresponding samples of the down sampledfirst reconstructed luma block using the linear model and theparameters.

In one aspect, a digital system configured to perform luma-based chromaintra-prediction is provided that includes means for down sampling afirst reconstructed luma block of a largest coding unit (LCU), means forcomputing parameters α and β of a linear model using immediate topneighboring reconstructed luma samples and left neighboringreconstructed luma samples of the first reconstructed luma block andreconstructed neighboring chroma samples of a chroma block correspondingto the first reconstructed luma block, wherein the linear model isPred_(C)[x,y]=α·Rec_(L)′[x,y]+β, wherein x and y are sample coordinates,Pred_(C) is predicted chroma samples, and Rec_(L)′ is samples of thedown sampled first reconstructed luma block, and wherein the immediatetop neighboring reconstructed luma samples are the only top neighboringreconstructed luma samples used, and means for computing samples of afirst predicted chroma block from corresponding samples of the downsampled first reconstructed luma block using the linear model and theparameters.

In one aspect, a computer readable medium is provided that storessoftware instructions that when executed in a digital system cause thedigital system to perform a method for luma-based chromaintra-prediction. The method includes down sampling a firstreconstructed luma block of a largest coding unit (LCU), computingparameters α and β of a linear model using immediate top neighboringreconstructed luma samples and left neighboring reconstructed lumasamples of the first reconstructed luma block and reconstructedneighboring chroma samples of a chroma block corresponding to the firstreconstructed luma block, wherein the linear model isPred_(C)[x,y]=α·Rec_(L)′[x,y]+β, wherein x and y are sample coordinates,Pred_(C) is predicted chroma samples, and Rec_(L)′ is samples of thedown sampled first reconstructed luma block, and wherein the immediatetop neighboring reconstructed luma samples are the only top neighboringreconstructed luma samples used, and computing samples of a firstpredicted chroma block from corresponding samples of the down sampledfirst reconstructed luma block using the linear model and theparameters.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments will now be described, by way of example only,and with reference to the accompanying drawings:

FIG. 1 illustrates prior art parameter derivation for luma-based chromaintra-prediction;

FIG. 2 illustrates prior art down sampling of a reconstructed luma blockand neighboring samples for luma-based chroma intra-prediction;

FIG. 3 is an example illustrating prior art luma-based chromaintra-prediction;

FIG. 4 illustrates prior art luma sample padding;

FIG. 5 illustrates down sampling of a reconstructed luma block andneighboring samples for luma-based chroma intra-prediction;

FIG. 6 illustrates luma sample padding;

FIG. 7 is a block diagram of a digital system;

FIG. 8 is a block diagram of a video encoder;

FIG. 9 is a block diagram of a video decoder;

FIG. 10 is an example illustrating use of a line buffer in luma-basedchroma intra-prediction;

FIGS. 11-13 are flow diagrams of methods; and

FIG. 14 is a block diagram of an illustrative digital system.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Specific embodiments of the invention will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency.

As used herein, the term “picture” may refer to a frame or a field of aframe. A frame is a complete image captured during a known timeinterval. For convenience of description, embodiments of the inventionare described herein in reference to HEVC. One of ordinary skill in theart will understand that embodiments of the invention are not limited toHEVC. In HEVC, a largest coding unit (LCU) is the base unit used forblock-based coding. A picture is divided into non-overlapping LCUs. Thatis, an LCU plays a similar role in coding as the macroblock ofH.264/AVC, but it may be larger, e.g., 32×32, 64×64, etc. An LCU may bepartitioned into coding units (CU). A CU is a block of pixels within anLCU and the CUs within an LCU may be of different sizes. Thepartitioning is a recursive quadtree partitioning. The quadtree is splitaccording to various criteria until a leaf is reached, which is referredto as the coding node or coding unit. The maximum hierarchical depth ofthe quadtree is determined by the size of the smallest CU (SCU)permitted. The coding node is the root node of two trees, a predictiontree and a transform tree. A prediction tree specifies the position andsize of prediction units (PU) for a coding unit. A transform treespecifies the position and size of transform units (TU) for a codingunit. A transform unit may not be larger than a coding unit and the sizeof a transform unit may be 4×4, 8×8, 16×16, and 32×32. The sizes of thetransforms units and prediction units for a CU are determined by thevideo encoder during prediction based on minimization of rate/distortioncosts.

For simplicity of explanation, a 4:2:0 sampling rate is assumed in whichfor each 2×2 luma sample, there are two corresponding chroma samples.Further, the following is assumed for intra-prediction of a CU. A CU canhave one PU, i.e., the CU and the PU are the same, or four PUs. Inaddition, a TU may be no larger than a PU. More specifically, a CU maybe divided into multiple TUs. If the CU is divided into four PUs, a TUmay be no larger than these PUs. However, the four PUs can each bedivided into multiple smaller TUs. If the CU is not subdivided, a TU canbe no larger than the CU and the CU can also be divided into multiplesmaller TUs. Each TU designates a luma block and the correspondingchroma block. That is, each luma block of a PU is the same size as andis in the same location in the PU as a TU. The size of a chroma blockfor a given luma block/TU is one half that of the luma block/TU. One ofordinary skill in the art, given the benefit of the description herein,will understand embodiments in which luma and chroma blocks in a blockof video data may be derived differently for intra-prediction.

Some aspects of this disclosure have been presented to the JCT-VC in M.Budagavi and A. Osamoto, “Luma-Based Chroma Intra PredictionSimplification”, JCTVC-F233, Joint Collaborative Team on Video Coding(JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, Torino, IT, Jul.14-22, 2011, which is incorporated by reference herein in its entirety.

As previously discussed, luma-based chroma intra-prediction, alsoreferred to as linear method mode (LM mode), is a new coding toolproposed in HEVC. A prior art version of LM mode intra-prediction isdescribed in detail in Weigand, et al., “WD3: Working Draft 3 ofHigh-efficiency Video Coding,” JCTVC-603, Mar. 16-23, 2011, Geneva, CH(“WD3” herein). In general, in luma-based chroma intra-prediction,chroma values for a block of video data, e.g., a TU, are predicted fromdown sampled reconstructed luma samples of the same block as follows:

Pred_(C) [x,y]=α·Rec_(L) ′[x,y]+β  (1)

where Pred_(C) is the predicted chroma samples and Rec_(L)′ is the downsampled reconstructed luma samples. In essence, the predicted chromasamples are modeled as a linear function of reconstructed luma samplesof the luma block. The reconstructed luma samples are down sampled tomatch the size and phase of the chroma samples. Down sampling, alsoreferred to as subsampling, is the process of reducing the sampling rateof a signal and is usually done to reduce the size of the data.

If Rec_(L) is the reconstructed luma samples in a block, Rec_(L)′ iscalculated as follows:

Rec_(L) ′[x,y]=(Rec_(L)[2x,2y]+Rec_(L)[2x,2y+1])>>1.  (2)

Parameters α and β are derived from reconstructed samples neighboringthe current chroma and reconstructed luma blocks. FIG. 1 shows anexample of the locations of these neighboring samples for an N×N chromablock and a 2N×2N reconstructed luma block in the prior art. In thisfigure, the shaded circles illustrate the positions of the reconstructedneighboring chroma and the down sampled reconstructed neighboring lumasamples, RecN_(C) and RecN_(L), around the respective chroma andreconstructed luma block.

The ordinary least square (OLS) technique, also referred to as thelinear least squares technique, is used to derive the parameters α andβ:

$\begin{matrix}{\alpha = {\frac{{I{\sum\limits_{i = 0}^{I - 1}{{{RecN}_{C}(i)}{{RecN}_{L}(i)}}}} - {\sum\limits_{i = 0}^{I - 1}{{{RecN}_{C}(i)}{\sum\limits_{i = 0}^{I - 1}{{RecN}_{L}(i)}}}}}{{I{\sum\limits_{i = 0}^{I - 1}{{{RecN}_{L}(i)}{{RecN}_{L}(i)}}}} - \left( {\sum\limits_{i = 0}^{I - 1}{{RecN}_{L}(i)}} \right)^{2}} = \frac{A_{1}}{A_{2\;}}}} & (3) \\{\mspace{79mu} {\beta = \frac{{\sum\limits_{i = 0}^{I - 1}{{RecN}_{C}(i)}} - {\alpha \; {\sum\limits_{i = 0}^{I - 1}{{RecN}_{L}(i)}}}}{I}}} & (4)\end{matrix}$

where RecN_(C)(i) and RecN_(L)(i) are, respectively, the neighboringreconstructed chroma samples and the down sampled reconstructedneighboring luma samples and I is the total number of samples of theneighboring data. For a target N×N chroma block, when both left and topcausal samples are available, the total number of involved samples is2N. When only the left or top causal samples are available, the totalnumber of involved samples is N.

In the prior art, e.g., WD3, for LM mode intra-prediction, the samplesof the reconstructed luma block and the neighboring reconstructed lumasamples are down sampled as shown in FIG. 2. In general, for the downsampling, pairs of pixels in every other column are averaged. Eachshaded circle represents the average of the two samples it covers. Thisdown sampling requires 2N second left neighboring reconstructed lumasamples, 2N immediate top neighboring reconstructed luma samples, and 2Nsecond top neighboring reconstructed luma samples. Using both theimmediate top neighboring reconstructed luma samples and the second topneighboring reconstructed luma samples requires that these 4N samples beretained in memory when the PUs containing the samples are reconstructedin an encoder or a decoder. Further, because LCUs in a picture areencoded and decoded in row order, the bottom two rows of reconstructedluma samples from each row of LCUs in a picture except the last row needto be stored in memory for use in LM mode intra-prediction of the topPUs in the LCU row.

More specifically, consider the simple two LCU row example of FIG. 3. Inthis example, LCU 0 and LCU 1 are in one row of LCUs of a picture andLCU 2 and LCU 3 are in the next row of LCUs. Each LCU is divided intofour CUs, CU0, CU1, CU2, and CU3. To support LM mode intra-prediction ofPU0, PU1, and PU4 of LCU 0, PU0, PU1 and PU2 of LCU 1, PU0, PU1 and PU2of LCU 2, and PU0, PU1, and PU4 of LCU 3, the last two rows ofreconstructed luma samples of the PUs immediately above these PUs areneeded. For example, for PU8 of LCU 0, the last two rows ofreconstructed luma samples of PU6 are needed for LM modeintra-prediction of PU8. To support LM mode intra-prediction of PU0,PU1, and PU2 of LCU 2 and PU0, PU1, and PU4 of LCU 3, the last two rowsof reconstructed luma samples of the PUs immediately above these PUS areneeded. For example, for PU2 of LCU 2, the last two rows ofreconstructed luma samples of PU9 in LCU 0 are needed for LM modeintra-prediction of PU2. Because the LCUs are encoded and decoded in roworder, both LCU 0 and LCU 1 will be processed before LCU 2 and LCU 3. Inorder to have the reconstructed luma samples needed for LM modeintra-prediction of PU0, PU1, and PU2 of LCU 2 and PU0, PU1, and PU4 ofLCU 3, the entire bottom two rows of reconstructed luma samples from LCU0 and LCU 1 need to be stored.

Therefore, sufficient memory is needed to store two picture wide rows ofreconstructed luma samples for LM mode intra-prediction. The amount ofmemory needed is significant given the large picture resolutionssupported by HEVC. For example, picture resolutions at least as large as8K×4K will be supported in the final standard. For embedded codecs suchas those used in cellular telephones, digital cameras, set-top boxes,and other consumer devices, memory, especially the on-chip memory thatwould be preferred to meet throughput requirements, is expensive.

The memory used to store a row of neighboring reconstructed luma samplesmay be referred to as a line buffer. Accordingly, two line buffers areneeded to store the two picture wide rows of reconstructed luma samples.A line buffer may be a specialized memory circuit designated for storageof a row of samples or may be a portion of memory in a larger memorycircuit that is used to store a row of samples.

HEVC also defines a mode referred to as constrained intra-predictionmode that affects all intra-prediction modes. When this mode is enabled,samples from inter-predicted PUs cannot be used for intra-prediction inany intra-prediction mode. Instead, sample padding, also referred to assample substitution, is used to supply values to replace theinter-predicted samples. For LM mode intra-prediction, the samplesubstitution has to be performed for both the immediate top neighboringreconstructed luma samples and the second top neighboring reconstructedluma samples as well as for the left neighboring reconstructed lumasamples to replace any sample values from inter-predicted PUs. FIG. 4shows a simple example of the prior art sample padding of luma samples.In this example, the samples designated as “not available” are frominter-predicted PUs. Sample padding is also performed on the left andtop neighboring reconstructed chroma samples. A detailed explanation ofthe prior art sample substitution is available in WD3.

This sample substitution process increases the time needed to performintra-prediction which may make it difficult to meet throughputrequirements. For example, in some pipelined codec architectures,intra-prediction, transformation, and quantization are performed in thesame pipeline stage, so any increase in the time needed to performintra-prediction is a concern.

Embodiments of the invention provide for simplification of the abovedescribed luma-based chroma intra-prediction by eliminating the use ofthe second top neighboring reconstructed luma samples in the derivationof the parameters α and β for LM mode intra-prediction of PUs at the topboundaries of LCUs. In some embodiments, the use of the second topneighboring reconstructed luma samples in the derivation of theparameters α and β for LM mode intra-prediction is eliminated for allPUs.

Eliminating the use of the second top neighboring reconstructed samplesfor LM mode intra-prediction of the PUs at the top boundaries of LCUseliminates the need to store a picture wide row of these samples, thusreducing the memory needed to support LM mode intra-prediction. It alsosimplifies the down sampling computation. FIG. 5 shows an example of thedown sampling when the immediate top neighboring reconstructed lumasamples are used. The black circles represent the down sampled immediatetop neighboring reconstructed luma samples. No averaging computation isneeded to down sample these luma samples. Rather, the samples in everyother column may be used. The down sampling for the remainingreconstructed luma samples remains the same and the shaded circlesrepresent the down sampled values.

Eliminating the use of the second top neighboring reconstructed samplesalso reduces the number of samples that may need to be padded. FIG. 6shows a simple example of luma sample padding when only the immediatetop neighboring reconstructed luma samples are used instead of usingboth the immediate top and second top neighboring samples. In thisexample, the samples designated as “not available” are frominter-predicted PUs. The video coding standard in use, e.g., HEVC,defines how the sample padding is performed. A detailed description ofone technique for sample padding when only the immediate top neighboringreconstructed luma samples are used can be found in Bross, et al., “HighEfficiency Video Coding (HEVC) Text Specification Draft 7,”JCTVC-I1003_d4, Apr. 27-May 7, 2012, Geneva, CH (“WD7” herein), which isincorporated by reference herein in its entirety.

FIG. 7 shows a block diagram of a digital system that includes a sourcedigital system 700 that transmits encoded video sequences to adestination digital system 702 via a communication channel 716. Thesource digital system 700 includes a video capture component 704, avideo encoder component 706, and a transmitter component 708. The videocapture component 704 is configured to provide a video sequence to beencoded by the video encoder component 706. The video capture component704 may be, for example, a video camera, a video archive, or a videofeed from a video content provider. In some embodiments, the videocapture component 704 may generate computer graphics as the videosequence, or a combination of live video, archived video, and/orcomputer-generated video.

The video encoder component 706 receives a video sequence from the videocapture component 704 and encodes it for transmission by the transmittercomponent 708. The video encoder component 706 receives the videosequence from the video capture component 704 as a sequence of pictures,divides the pictures into largest coding units (LCUs), and encodes thevideo data in the LCUs. The video encoder component 706 may beconfigured to apply luma-based chroma intra-prediction techniques duringthe encoding process as described herein. An embodiment of the videoencoder component 706 is described in more detail herein in reference toFIG. 8.

The transmitter component 708 transmits the encoded video data to thedestination digital system 702 via the communication channel 716. Thecommunication channel 716 may be any communication medium, orcombination of communication media suitable for transmission of theencoded video sequence, such as, for example, wired or wirelesscommunication media, a local area network, or a wide area network.

The destination digital system 702 includes a receiver component 710, avideo decoder component 712 and a display component 714. The receivercomponent 710 receives the encoded video data from the source digitalsystem 700 via the communication channel 716 and provides the encodedvideo data to the video decoder component 712 for decoding. The videodecoder component 712 reverses the encoding process performed by thevideo encoder component 706 to reconstruct the LCUs of the videosequence. The video decoder component 712 may be configured to applyluma-based chroma intra-prediction techniques during the decodingprocess as described herein. An embodiment of the video decodercomponent 712 is described in more detail below in reference to FIG. 9.

The reconstructed video sequence is displayed on the display component714. The display component 714 may be any suitable display device suchas, for example, a plasma display, a liquid crystal display (LCD), alight emitting diode (LED) display, etc.

In some embodiments, the source digital system 700 may also include areceiver component and a video decoder component and/or the destinationdigital system 702 may include a transmitter component and a videoencoder component for transmission of video sequences both directionsfor video steaming, video broadcasting, and video telephony. Further,the video encoder component 706 and the video decoder component 712 mayperform encoding and decoding in accordance with one or more videocompression standards. The video encoder component 706 and the videodecoder component 712 may be implemented in any suitable combination ofsoftware, firmware, and hardware, such as, for example, one or moredigital signal processors (DSPs), microprocessors, discrete logic,application specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), etc.

FIG. 8 shows a block diagram of the LCU processing portion of an examplevideo encoder. A coding control component (not shown) sequences thevarious operations of the LCU processing, i.e., the coding controlcomponent runs the main control loop for video encoding. The codingcontrol component receives a digital video sequence and performs anyprocessing on the input video sequence that is to be done at the picturelevel, such as determining the coding type (I, P, or B) of a picturebased on the high level coding structure, e.g., IPPP, IBBP,hierarchical-B, and dividing a picture into LCUs for further processing.The coding control component also may determine the initial LCU CUstructure for each CU and provides information regarding this initialLCU CU structure to the various components of the video encoder asneeded. The coding control component also may determine the initial PUand TU structure for each CU and provides information regarding thisinitial structure to the various components of the video encoder asneeded.

In addition, for pipelined architectures in which multiple PUs and CUsmay be processed concurrently in different components of the LCUprocessing, the coding control component controls the processing of PUsand CUs by various components of the LCU processing in a pipelinefashion. For example, in many embedded systems supporting videoprocessing, there may be one master processor and one or more slaveprocessing modules, e.g., hardware accelerators. The master processoroperates as the coding control component and runs the main control loopfor video encoding, and the slave processing modules are employed to offload certain compute-intensive tasks of video encoding such as motionestimation, motion compensation, intra prediction mode estimation,transformation and quantization, entropy coding, and loop filtering. Theslave processing modules are controlled in a pipeline fashion by themaster processor such that the slave processing modules operate ondifferent blocks of a picture at any given time. That is, the slaveprocessing modules are executed in parallel, each processing itsrespective block while data movement from one processor to another isserial.

The LCU processing receives LCUs of the input video sequence from thecoding control component and encodes the LCUs under the control of thecoding control component to generate the compressed video stream. TheLCUs in each picture are processed in row order. The CUs in the CUstructure of an LCU may be processed by the LCU processing in adepth-first Z-scan order. The LCUs 800 from the coding control unit areprovided as one input of a motion estimation component 820, as one inputof an intra-prediction component 824, and to a positive input of acombiner 802 (e.g., adder or subtractor or the like). Further, althoughnot specifically shown, the prediction mode of each picture as selectedby the coding control component is provided to a mode selector componentand the entropy encoder 834.

The storage component 818 provides reference data to the motionestimation component 820 and to the motion compensation component 822.The reference data may include one or more previously encoded anddecoded CUs, i.e., reference CUs.

The motion estimation component 820 provides motion data information tothe motion compensation component 822 and the entropy encoder 834. Morespecifically, the motion estimation component 820 performs tests on CUsin an LCU based on multiple inter-prediction modes (e.g., skip mode,merge mode, and normal or direct inter-prediction) and transform blocksizes using reference picture data from storage 818 to choose the bestmotion vector(s)/prediction mode based on a rate distortion coding cost.To perform the tests, the motion estimation component 820 may begin withthe CU structure provided by the coding control component. The motionestimation component 820 may divide each CU indicated in the CUstructure into PUs according to the unit sizes of prediction modes andinto transform units according to the transform block sizes andcalculate the coding costs for each prediction mode and transform blocksize for each CU. The motion estimation component 820 may also computeCU structure for the LCU and PU/TU partitioning structure for a CU ofthe LCU by itself.

For coding efficiency, the motion estimation component 820 may alsodecide to alter the CU structure by further partitioning one or more ofthe CUs in the CU structure. That is, when choosing the best motionvectors/prediction modes, in addition to testing with the initial CUstructure, the motion estimation component 820 may also choose to dividethe larger CUs in the initial CU structure into smaller CUs (within thelimits of the recursive quadtree structure), and calculate coding costsat lower levels in the coding hierarchy. If the motion estimationcomponent 820 changes the initial CU structure, the modified CUstructure is communicated to other components that need the information.

The motion estimation component 820 provides the selected motion vector(MV) or vectors and the selected prediction mode for eachinter-predicted PU of a CU to the motion compensation component 822 andthe selected motion vector (MV), reference picture index (indices),prediction direction (if any) to the entropy encoder 834

The motion compensation component 822 provides motion compensatedinter-prediction information to the mode decision component 826 thatincludes motion compensated inter-predicted PUs, the selectedinter-prediction modes for the inter-predicted PUs, and correspondingtransform block sizes. The coding costs of the inter-predicted PUs arealso provided to the mode decision component 826.

The intra-prediction component 824 provides intra-prediction informationto the mode decision component 826 that includes intra-predicted PUs andthe corresponding intra-prediction modes. That is, the intra-predictioncomponent 824 performs intra-prediction in which tests based on multipleintra-prediction modes and transform unit sizes are performed on CUs inan LCU using previously encoded neighboring PUs from the buffer 828 tochoose the best intra-prediction mode for each PU in the CU based on acoding cost. As is well known, a block of video data includes both lumadata and chroma data, which may be encoded separately. Differentintra-prediction modes may be used for luma blocks and chroma blocks ina PU.

One of the intra-prediction modes considered by the intra-predictioncomponent 824 for chroma is a luma-based chroma intra-prediction mode,i.e., LM mode intra-prediction. The luma-based chroma intra-predictionmay be performed according to methods described herein. As is describedin more detail herein, the LM mode intra-prediction uses immediate topneighboring reconstructed luma samples and second left neighboringreconstructed luma samples to derive the parameters α and β for LM modeintra-prediction of PUs at the top boundaries of LCUs. These immediatetop neighboring reconstructed samples are stored in a portion of thebuffer 828 designated for storing a picture wide row of such samples.This portion of the buffer 828 is referred to as a top neighboringsample line buffer herein. The second left neighboring reconstructedluma samples are also stored in the buffer 828.

In some embodiments, LM mode intra-prediction of PUs not at the topboundaries of LCUs also uses immediate top neighboring reconstructedluma samples and second left neighboring reconstructed luma samples toderive the parameters α and β. In some embodiments, LM modeintra-prediction of PUs not at the top boundaries of LCUs uses bothimmediate top neighboring reconstructed luma samples and second topneighboring reconstructed luma samples together with the second leftneighboring reconstructed luma samples to derive the parameters α and β.The reconstructed neighboring samples needed for LM modeintra-prediction of the PUs not at the top boundaries of LCUs are alsostored in the buffer 828.

If constrained intra-prediction mode is enabled, the intra-predictioncomponent 824 performs sample padding as needed before using thereconstructed neighboring luma and chroma samples to derive theparameters α and β. In general, the sample padding replaces any samplesin the reconstructed neighboring samples that are from inter-predictedPUs with sample values better suited for intra-prediction. The samplepadding process is defined by the video coding standard in use, e.g.,HEVC. A detailed description of one suitable technique for samplepadding is described in WD7.

To perform the tests, the intra-prediction component 824 may begin withthe CU structure provided by the coding control. The intra-predictioncomponent 824 may divide each CU indicated in the CU structure into PUsaccording to the unit sizes of the intra-prediction modes and intotransform units according to the transform block sizes and calculatesthe coding costs for each prediction mode and transform block size foreach PU. For coding efficiency, the intra-prediction component 824 mayalso decide to alter the CU structure by further partitioning one ormore of the CUs in the CU structure. That is, when choosing the bestprediction modes, in addition to testing with the initial CU structure,the intra-prediction component 824 may also chose to divide the largerCUs in the initial CU structure into smaller CUs (within the limits ofthe recursive quadtree structure), and calculate coding costs at lowerlevels in the coding hierarchy. If the intra-prediction component 824changes the initial CU structure, the modified CU structure iscommunicated to other components that need the information. Further, thecoding costs of the intra-predicted PUs and the associated transformblock sizes are also provided to the mode decision component 826.

The mode decision component 826 selects between the motion-compensatedinter-predicted PUs from the motion compensation component 822 and theintra-predicted PUs from the intra-prediction component 824 based on thecoding costs of the PUs and the picture prediction mode provided by themode selector component. The decision is made at CU level. Based on thedecision as to whether a CU is to be intra- or inter-coded, theintra-predicted PUs or inter-predicted PUs are selected, accordingly.

The output of the mode decision component 826, i.e., the predicted PU,is provided to a negative input of the combiner 802 and to a delaycomponent 830. The associated transform block size is also provided tothe transform component 804. The output of the delay component 830 isprovided to another combiner (i.e., an adder) 838. The combiner 802subtracts the predicted PU from the current PU to provide a residual PUto the transform component 804. The resulting residual PU is a set ofpixel difference values that quantify differences between pixel valuesof the original PU and the predicted PU. The residual blocks of all thePUs of a CU form a residual CU block for the transform component 804.

The transform component 804 performs block transforms on the residual CUto convert the residual pixel values to transform coefficients andprovides the transform coefficients to a quantize component 806. Thetransform component 804 receives the transform block sizes for theresidual CU and applies transforms of the specified sizes to the CU togenerate transform coefficients.

The quantize component 806 quantizes the transform coefficients based onquantization parameters (QPs) and quantization matrices provided by thecoding control component and the transform sizes. The quantizedtransform coefficients are taken out of their scan ordering by a scancomponent 808 and arranged by significance, such as, for example,beginning with the more significant coefficients followed by the lesssignificant.

The ordered quantized transform coefficients for a CU provided via thescan component 808 along with header information for the CU are coded bythe entropy encoder 834, which provides a compressed bit stream to avideo buffer 836 for transmission or storage. The header information mayinclude the prediction mode used for the CU. The entropy encoder 834also encodes the CU and PU structure of each LCU.

The LCU processing includes an embedded decoder. As any compliantdecoder is expected to reconstruct an image from a compressed bitstream, the embedded decoder provides the same utility to the videoencoder. Knowledge of the reconstructed input allows the video encoderto transmit the appropriate residual energy to compose subsequentpictures. To determine the reconstructed input, i.e., reference data,the ordered quantized transform coefficients for a CU provided via thescan component 808 are returned to their original post-transformarrangement by an inverse scan component 810, the output of which isprovided to a dequantize component 812, which outputs a reconstructedversion of the transform result from the transform component 804.

The dequantized transform coefficients are provided to the inversetransform component 814, which outputs estimated residual informationrepresenting a reconstructed version of a residual CU. The inversetransform component 814 receives the transform block size used togenerate the transform coefficients and applies inverse transform(s) ofthe specified size to the transform coefficients to reconstruct theresidual values.

The reconstructed residual CU is provided to the combiner 838. Thecombiner 838 adds the delayed selected CU to the reconstructed residualCU to generate a reconstructed CU, which becomes part of reconstructedpicture data. The reconstructed picture data is stored in a buffer 828for use by the intra-prediction component 824 and is provided to anin-loop filter component 816. When the reconstructed CU is a CU at thebottom boundary of an LCU, the last row of reconstructed samples in theCU is also stored in the top neighboring sample line buffer in thebuffer 828 for use by the intra-prediction component 824 in LM modeintra-prediction of the PUs at the top boundaries of LCUs in the nextLCU row.

The in-loop filter component 816 applies various filters to thereconstructed picture data to generate final reference picture data. Thefiltering may be performed, for example, on an LCU-by-LCU basis or on apicture basis. This filtering is performed to improve the referencepictures used for encoding/decoding of subsequent pictures. The in-loopfilter component 816 may, for example, adaptively apply low-pass filtersto block boundaries according to the boundary strength to alleviateblocking artifacts causes by the block-based video coding. The finalreference picture data is provided to storage component 818.

FIG. 9 shows a block diagram of an example video decoder. The videodecoder operates to reverse the encoding operations, i.e., entropycoding, quantization, transformation, and prediction, performed by thevideo encoder of FIG. 8 to regenerate the pictures of the original videosequence. In view of the above description of a video encoder, one ofordinary skill in the art will understand the functionality ofcomponents of the video decoder without detailed explanation.

The entropy decoding component 900 receives an entropy encoded(compressed) video bit stream and reverses the entropy coding to recoverthe encoded PUs and header information such as the prediction modes andthe encoded CU and PU structures of the LCUs. If the decoded predictionmode is an inter-prediction mode, the entropy decoder 900 thenreconstructs the motion vector(s) as needed and provides the motionvector(s) to the motion compensation component 910.

The inverse quantization component 902 de-quantizes the quantizedtransform coefficients of the residual CU. The inverse transformcomponent 904 transforms the frequency domain data from the inversequantization component 902 back to the residual CU. That is, the inversetransform component 904 applies an inverse unit transform, i.e., theinverse of the unit transform used for encoding, to the de-quantizedresidual coefficients to produce the residual CUs.

A residual CU supplies one input of the addition component 906. Theother input of the addition component 906 comes from the mode switch908. When an inter-prediction mode is signaled in the encoded videostream, the mode switch 908 selects predicted PUs from the motioncompensation component 910 and when an intra-prediction mode issignaled, the mode switch selects predicted PUs from theintra-prediction component 914.

The motion compensation component 910 receives reference data fromstorage 912 and applies the motion compensation computed by the encoderand transmitted in the encoded video bit stream to the reference data togenerate a predicted PU. That is, the motion compensation component 910uses the motion vector(s) from the entropy decoder 900 and the referencedata to generate a predicted PU.

The intra-prediction component 914 receives reconstructed samples frompreviously reconstructed PUs of a current picture from the buffer 907and performs the intra-prediction computed by the encoder as signaled byan intra-prediction mode transmitted in the encoded video bit streamusing the reconstructed samples as needed to generate a predicted PU. IfLM mode intra-prediction is signaled, the intra-prediction component 914may perform the luma-based chroma intra-prediction according to methodsdescribed herein.

As is described in more detail herein, the LM mode intra-prediction usesimmediate top neighboring reconstructed luma samples and second leftneighboring reconstructed luma samples to derive the parameters α and βfor LM mode intra-prediction of PUs at the top boundaries of LCUs. Theseimmediate top neighboring reconstructed samples are stored in a portionof the buffer 907 designated for storing a picture wide row of suchsamples. This portion of the buffer 907 is referred to as a topneighboring sample line buffer herein. The second left neighboringreconstructed luma samples are also stored in the buffer 907.

In some embodiments, LM mode intra-prediction of PUs not at the topboundaries of LCUs also uses immediate top neighboring reconstructedluma samples and second left neighboring reconstructed luma samples toderive the parameters α and β. In some embodiments, LM modeintra-prediction of PUs not at the top boundaries of LCUs uses bothimmediate top neighboring reconstructed luma samples and second topneighboring reconstructed luma samples together with the second leftneighboring reconstructed luma samples to derive the parameters α and β.The reconstructed neighboring samples needed for LM modeintra-prediction of the PUs not at the top boundaries of LCUs are alsostored in the buffer 907. If constrained intra-prediction mode isenabled, the intra-prediction component 914 performs sample padding asneeded before using the reconstructed neighboring samples to derive theparameters α and β. The sample padding is the same as that performed inthe encoder.

The addition component 906 generates a reconstructed CU by adding thepredicted PUs selected by the mode switch 908 and the residual CU. Theoutput of the addition component 906, i.e., the reconstructed CUs,supplies the input of the in-loop filter component 916 and is alsostored in the buffer 907 for use by the intra-prediction component 914.When the reconstructed CU is a CU at the bottom boundary of an LCU, thelast row of reconstructed samples in the CU is also stored in the topneighboring sample line buffer in the buffer 907 for use by theintra-prediction component 914 in LM mode intra-prediction of PUs at thetop boundaries of LCUs in the next LCU row.

The in-loop filter component 916 performs the same filtering as theencoder. The output of the in-loop filter component 916 is the decodedpictures of the video bit stream. Further, the output of the in-loopfilter component 916 is stored in storage 912 to be used as referencedata by the motion compensation component 908.

Embodiments of the encoder and the decoder may be implemented inhardware, software, firmware, or any combination thereof. In someembodiments, the buffers 828 and 907 used to store reconstructed samplesfor intra-prediction may be implemented in an on-chip memory of anembedded codec. In some such embodiments, the top neighboring sampleline buffers may be implemented as a portion of this on-chip memory. Insome such embodiments, the top neighboring sample line buffers may beimplemented as on-chip hardware line buffers.

If embodiments are completely or partially implemented in software, thesoftware may be executed in one or more processors, such as amicroprocessor, application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA), or digital signal processor (DSP). Thesoftware instructions may be initially stored in a computer-readablemedium and loaded and executed in the processor. In some cases, thesoftware instructions may also be sold in a computer program product,which includes the computer-readable medium and packaging materials forthe computer-readable medium. In some cases, the software instructionsmay be distributed via removable computer readable media, via atransmission path from computer readable media on another digitalsystem, etc. Examples of computer-readable media include non-writablestorage media such as read-only memory devices, writable storage mediasuch as disks, flash memory, memory, or a combination thereof.

FIG. 10 shows a simplified example illustrating the use of a topneighboring sample line buffer in an encoder or a decoder. The CU and PUstructure of each LCU is depicted as being identical for simplicity ofexplanation. One of ordinary skill in the art will understand that CUand PU structures may vary and are not necessarily the same for eachLCU. In this example, two LCU rows of a picture are depicted, LCU Row 0and LCU Row 1. As the LCUs of LCU Row 0 are reconstructed in an encoderor decoder, the last row of samples in each reconstructed CU at thebottom boundary of each LCU in the row, i.e., CU2 and CU3 of each of LCU0, LCU 1, and LCU 2, is stored in the sample line buffer after each CUis reconstructed. The reconstructed samples are stored in the linebuffer for both intra-predicted and inter-predicted CUs.

Then, as the LCUs of Row 1 are processed, and assuming that constrainedintra-prediction mode is not enabled, the reconstructed samples in theline buffer are used for LM mode intra-prediction of the PUs at the topboundary of each LCU in the row, i.e., PU0, PU1, and PU4 of each of LCU0, LCU 1, and LCU 2. For each of these PUs, the reconstructed samplesthat correspond to the immediate top neighboring reconstructed samplesof the PU are read from the line buffer. For example, in this simpleexample, for LM mode prediction of PU0 of CU0 of LCU 0, a portion of thereconstructed samples from the last row of PU5 of CU2 of LCU 0 of LCURow 0 are read. In another example, for LM mode prediction of PU4 of CU0of LCU 0, the reconstructed samples from the last rows of PU8 and PU9 ofCU3 of LCU 0 of LCU Row 0 are read. In an encoder, since LM modeintra-prediction is considered for all PUs, all of the reconstructedsamples in the line buffer will be used. In a decoder, reconstructedsamples in the line buffer are used only for those PUs that were encodedusing LM mode intra-prediction.

If constrained intra-prediction mode is enabled, any reconstructedsamples in the line buffer that are from inter-predicted PUs cannot beused for LM mode intra-prediction (or for any other intra-predictionmode). In such cases, prior to performing intra-prediction on a PU atthe top boundary of an LCU in LCU Row 1, sample padding is performed toreplace any reconstructed samples in the line buffer that came from aninter-predicted PU and may be used for intra-prediction of the PU withsamples more suitable for intra-prediction. For example, if PU8 and PU9of CU3 of LCU2 in LCU Row 0 are inter-predicted, then any reconstructedsamples from these two PUs will be replaced by the sample padding priorto performing intra-prediction on PU4 of LCU 2 in LCU Row 1.

FIG. 11 shows a method for LM mode intra-prediction of a chroma blockusing immediate top neighboring reconstructed luma samples and secondleft neighboring reconstructed luma samples. This method may be used inan encoder or a decoder. The interior samples of the reconstructed lumablock 1100 corresponding to the chroma block are filtered and downsampled to generate a filtered, down sampled luma block 1102 forgenerating the predicted chroma block. The immediate top neighboringreconstructed luma samples and second left neighboring reconstructedluma samples of the reconstructed luma block 1100 are also filtered anddown sampled for use in deriving the parameters α and β. In someembodiments, if the reconstructed luma block 1100 is at the top boundaryof an LCU, the immediate top neighboring reconstructed luma samples areaccessed from a top neighboring sample line buffer. In some suchembodiments, if the reconstructed luma block 1100 is not at the topboundary of an LCU, the immediate top neighboring reconstructed lumasamples are accessed from a buffer storing reconstructed blocks for usein intra-prediction.

Although not specifically shown, in some embodiments, intra-predictionmodes may optionally be constrained such that these modes are notallowed to use samples from inter-predicted blocks. In such embodiments,when the constraint is enabled, sample padding is performed on theimmediate top neighboring reconstructed luma samples and second leftneighboring reconstructed luma samples prior to the filtering and downsampling to replace any samples from inter-predicted blocks.

The parameters α and β are then derived using the top and leftneighboring reconstructed chroma samples, RecN_(C)(i), of thecorresponding chroma block 1104 and the filtered, down sampled immediatetop and second left neighboring reconstructed samples, RecN_(L)(i), ofthe luma block 1100 as per Eq. 3 and 4. The division operations may beapproximated using a look-up table (LUT). The parameters α and β arethen used to compute the predicted chroma values 1106 from the filtered,down sampled luma block 1102 as per Eq. 1.

Although not specifically shown, the parameter α may be normalized toreduce the precision of the parameter prior to computing the predictedchroma values. The normalization may be performed, for example, bycounting the leading number of zeroes (for positive numbers) or ones(for negative numbers) and shifting by the count to normalize the value.In one embodiment, the value of α is normalized as per the followingequation:

a=a3>>Max(0,Log 2(abs(a3))−6)

where a3 is the original value of α and a is the value of α afternormalization. The above equation may be implemented as per thefollowing pseudo code:

Short n=CountLeadingZerosOnes(a);

a=a>>(9−n);

Note that the above equation and pseudo code reduce the precision of αto 7 bits. Other normalization of a may also be used to reduce theprecision of α to, for example, 6 bits or 8 bits.

FIG. 12 shows a method for LM mode intra-prediction of a chroma block ofa PU from a corresponding reconstructed luma block in which onlyimmediate top neighboring reconstructed luma samples and second leftneighboring reconstructed luma samples are used. This method may be usedin an encoder or a decoder. Initially, a check 1200 is made to determineif the luma block is at the top boundary of the LCU. If the luma blockis at the top boundary, the immediate top neighboring reconstructed lumasamples are received 1202 from a top neighboring sample line bufferstoring the last row of reconstructed samples from the previous LCU row.If the luma block is not at the top boundary, the immediate topneighboring reconstructed luma samples are received 1204 from anintra-prediction buffer storing reconstructed LCUs for use inintra-prediction. The second left neighboring reconstructed luma samplesand the top and left neighboring reconstructed chroma samples are alsoreceived 1206 from the intra-prediction buffer.

If constrained intra-prediction mode is enabled 1208, sample padding1210 is performed on the immediate top neighboring reconstructed lumasamples, the second left neighboring reconstructed luma samples, and theneighboring reconstructed chroma samples to replace any samples frominter-predicted PUs with values better suited for intra-prediction. Thesample padding used is defined by the video coding standard in use.

The immediate top neighboring reconstructed luma samples and second leftneighboring reconstructed luma samples are filtered and down sampled1212 to reduce the sampling rate of the luma samples to that of thechroma samples. Down sampling of immediate top neighboring reconstructedluma samples is previously described herein. Any suitable filters may beused and the filters for top neighboring samples and left neighboringsamples may be different, e.g., [0.5, 0.5], [0.25, 0.5, 0.25] etc. Insome embodiments, the neighboring reconstructed chroma samples are alsofiltered. In such embodiments, different filters may be used for chromaand luma samples.

The parameters α and β are then computed 1214 by linear regression usingthe filtered, down sampled immediate top neighboring reconstructed lumasamples and second left neighboring reconstructed luma samples and theneighboring reconstructed chroma samples. Any suitable technique forlinear regression may be used. In some embodiments, the ordinary leastsquares technique of Eq. 3 and 4 is used. The two parameters are thenused to compute 1216 the predicted chroma block from the correspondingdown sampled reconstructed luma block as per Eq. 1. In some embodiments,the parameter α is normalized prior to computing the predicted chromablock. Normalization of the parameter α is previously described herein.

FIG. 13 shows a method for LM mode intra-prediction of a chroma block ofa PU from a corresponding reconstructed luma block. This method may beused in an encoder or a decoder. Initially, a check 1300 is made todetermine if the luma block is at the top boundary of the LCU. If theluma block is at the top boundary, the immediate top neighboringreconstructed luma samples are received 1302 from a top neighboringsample line buffer storing the last row of reconstructed samples fromthe previous LCU row. The second left neighboring reconstructed lumasamples and the top and left neighboring reconstructed chroma samplesare also received 1306 from the intra-prediction buffer.

If constrained intra-prediction mode is enabled 1308, sample padding1310 is performed on the immediate top neighboring reconstructed lumasamples, the second left neighboring reconstructed luma samples, and theneighboring reconstructed chroma samples to replace any samples frominter-predicted PUs with values better suited for intra-prediction. Thesample padding used is defined by the video coding standard in use.

The immediate top neighboring reconstructed luma samples and second leftneighboring reconstructed luma samples are filtered and down sampled1312 to reduce the sampling rate of the luma samples to that of thechroma samples. Down sampling of immediate top neighboring reconstructedluma samples is previously described herein. Any suitable filters may beused and the filters for top neighboring samples and left neighboringsamples may be different, e.g., [0.5, 0.5], [0.25, 0.5, 0.25] etc. Insome embodiments, the neighboring reconstructed chroma samples are alsofiltered. In such embodiments, different filters may be used for chromaand luma samples.

The parameters α and β are then computed 1314 by linear regression usingthe filtered, down sampled immediate top neighboring reconstructed lumasamples and second left neighboring reconstructed luma samples and theneighboring reconstructed chroma samples. Any suitable technique forlinear regression may be used. In some embodiments, the ordinary leastsquares technique of Eq. 3 and 4 is used. The two parameters are thenused to compute 1316 the predicted chroma block from the correspondingdown sampled reconstructed luma block as per Eq. 1. In some embodiments,the parameter α is normalized prior to computing the predicted chromablock. Normalization of the parameter α is previously described herein.

If the luma block is not at the top boundary, the immediate topneighboring reconstructed luma samples and the second top neighboringreconstructed luma samples are received 1304 from an intra-predictionbuffer storing reconstructed LCUs for use in intra-prediction. Thesecond left neighboring reconstructed luma samples and the top and leftneighboring reconstructed chroma samples are also received 1306 from theintra-prediction buffer.

If constrained intra-prediction mode is enabled 1308, sample padding1310 is performed on the immediate top neighboring reconstructed lumasamples, the second top neighboring reconstructed luma samples, thesecond left neighboring reconstructed luma samples, and the neighboringreconstructed chroma samples to replace any samples from inter-predictedPUs with values better suited for intra-prediction. The sample paddingused is defined by the video coding standard in use.

The immediate top neighboring reconstructed luma samples, the second topneighboring reconstructed luma samples and second left neighboringreconstructed luma samples are filtered and down sampled 1312 to reducethe sampling rate of the luma samples to that of the chroma samples.Down sampling of immediate top neighboring reconstructed luma samplesand second top neighboring reconstructed luma samples is previouslydescribed herein. Any suitable filters may be used and the filters fortop neighboring samples and left neighboring samples may be different,e.g., [0.5, 0.5], [0.25, 0.5, 0.25] etc. In some embodiments, theneighboring reconstructed chroma samples are also filtered. In suchembodiments, different filters may be used for chroma and luma samples.

The parameters α and β are then computed 1314 by linear regression usingthe filtered, down sampled immediate top neighboring and second topneighboring reconstructed luma samples and second left neighboringreconstructed luma samples and the neighboring reconstructed chromasamples. Any suitable technique for linear regression may be used. Insome embodiments, the ordinary least squares technique of Eq. 3 and 4 isused. The two parameters are then used to compute 1316 the predictedchroma block from the corresponding down sampled reconstructed lumablock as per Eq. 1.

In some embodiments, the parameter α is normalized prior to computingthe predicted chroma block. Normalization of the parameter α ispreviously described herein.

In the prior art, both immediate top neighboring reconstructed lumasamples and second top neighboring reconstructed luma samples is usedfor all LM mode intra-prediction because a 4.2.0 sampling rate is used.When the luma samples are down sampled, the two rows of neighboringreconstructed luma samples correspond to one row of chroma samples,which provides somewhat better compression performance than using justthe immediate top neighboring reconstructed luma samples. However,simulations performed on multiple classes of test video sequences usingjust the immediate top neighboring reconstructed luma samples along withsecond left neighboring reconstructed luma samples for LM modeintra-prediction during encoding showed that there is a nominal bit rateincrease over the prior art.

The simulation results are summarized in Table 1. This table shows theincrease in bit rate for Y, U, and V over the prior art for five classesof test video sequences for two different test conditions, Intra andIntra LC. Under the Intra test conditions, all frames are encoded usingintra-predicted pictures only and with a goal of achieving highercompression performance. Under Intra LC test conditions, all frames areencoded using intra-predicted pictures only with a goal of low delay inthroughput. BD-Rate stands for Bjontegaard delta bit rate, which isdescribed in G. Bjontegaard, “Calculation of Average PSNR DifferencesBetween RD-Curves, Doc. VCEG-M33, ITU-T SC16/Q6, 13^(th) VCEG Meeting,Austin, Tex., April 2001. The Class A video sequences are 2560×1600sequences, the Class B video sequences are 1080p sequences, the Class Cvideo sequences are WVGA (Wide Video Graphics Array) sequences, theClass D video sequences are WQVGA (Wide Quarter Video Graphics Array)sequences, and the Class E video sequences are 720p sequences. Positivenumbers indicate an increase in bit rate.

As can be seen from this table, on average there is about 0.2% bit rateincrease for U and V over the prior art. This nominal increase may beacceptable in many codec applications in exchange for eliminating theneed to store two picture-wide rows of reconstructed luma samples.

TABLE 1 Intra Intra LC Y BD- U BD- V BD- Y BD- U BD- V BD- rate raterate rate rate rate Class A 0.0 0.6 0.7 0.0 0.7 0.6 Class B 0.0 0.1 0.10.0 0.1 0.0 Class C 0.0 0.2 0.2 0.0 0.1 0.2 Class D 0.0 0.1 0.1 0.0 0.10.1 Class E 0.0 0.0 0.1 0.0 0.0 0.0 All 0.0 0.2 0.2 0.0 0.2 0.2 EncTime[%] 97% 99% Dec Time[%] 96% 98%

Embodiments of the methods, encoders, and decoders described herein maybe implemented for virtually any type of digital system (e.g., a desktop computer, a laptop computer, a tablet computing device, a netbookcomputer, a handheld device such as a mobile (i.e., cellular) phone, apersonal digital assistant, a digital camera, a set top box, a digitalvideo recorder, etc.). FIG. 14 is a block diagram of a digital system1400 (e.g., a mobile cellular telephone) that may be configured to usetechniques described herein.

As shown in FIG. 14, the signal processing unit (SPU) 1402 includes adigital signal processing system (DSP) that includes embedded memory andsecurity features. The analog baseband unit 1404 receives a voice datastream from the handset microphone 1413 a and sends a voice data streamto the handset mono speaker 1413 b. The analog baseband unit 1404 alsoreceives a voice data stream from the microphone 1414 a or 1432 a andsends a voice data stream to the mono headset 1414 b or wireless headset1432 b. The analog baseband unit 1404 and the SPU 1402 may be separateICs. In many embodiments, the analog baseband unit 1404 does not embed aprogrammable processor core, but performs processing based onconfiguration of audio paths, filters, gains, etc being setup bysoftware running on the SPU 1402.

The display 1420 may display pictures and video sequences received froma local camera 1428, or from other sources such as the USB 1426 or thememory 1412. The SPU 1402 may also send a video sequence to the display1420 that is received from various sources such as the cellular networkvia the RF transceiver 1406 or the Bluetooth interface 1430. The SPU1402 may also send a video sequence to an external video display unitvia the encoder unit 1422 over a composite output terminal 1424. Theencoder unit 1422 may provide encoding according to PAL/SECAM/NTSC videostandards.

The SPU 1402 includes functionality to perform the computationaloperations required for video encoding and decoding. In one or moreembodiments, the SPU 1402 is configured to perform computationaloperations for applying one or more techniques for luma-based chromaintra-prediction during the encoding process as described herein.Software instructions implementing all or part of the techniques may bestored in the memory 1412 and executed by the SPU 1402, for example, aspart of encoding video sequences captured by the local camera 1428. TheSPU 1402 is also configured to perform computational operations forapplying one or more techniques for luma-based chroma intra-predictionas described herein as part of decoding a received coded video sequenceor decoding a coded video sequence stored in the memory 1412. Softwareinstructions implementing all or part of the techniques may be stored inthe memory 1412 and executed by the SPU 1402.

Other Embodiments

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.

For example, previously described embodiments assumed a 4:2:0 samplingrate. Other sampling rates may also be used, such as 4:2:2 and 4:4:4, aslong as the number of neighboring samples I is a power of 2. For a 4:4:4sampling rate, the initial down sampling of the luma block and theneighboring luma samples to the chroma sampling rate is not needed. Fora 4:2:2 sampling rate, the initial down sampling of the luma block andthe neighboring luma samples could be different in the horizontal andvertical directions. For example, for a 16×16 luma block, the chromablock would be 16×8 and the luma block would need to be down sampleddifferently in the horizontal and vertical directions.

In another example, although embodiments herein are described in whichthe left neighboring reconstructed luma samples are taken from thesecond column of reconstructed luma samples to the left of thereconstructed luma block, other left neighboring reconstructed lumasamples may be used, such as the immediate left neighboringreconstructed luma samples, i.e., those in the first column ofreconstructed samples to the left of the reconstructed luma block. Insuch embodiments, the down sampling of the immediate left neighboringreconstructed luma samples prior to computation of the parameters α andβ can be performed in a similar fashion to the down sampling of theimmediate top neighboring reconstructed luma samples.

In another example, if a 4:4:4 sampling rate is used, the immediate leftimmediate left neighboring reconstructed luma samples and the immediatetop neighboring reconstructed luma samples may be used for all LM modeintra-prediction since the sampling rates of the luma and the chroma arethe same. In such embodiments, the luma block and the neighboringreconstructed luma samples are not down sampled unless the neighboringreconstructed chroma samples and the chroma block are identically downsampled.

Embodiments of the methods described herein may be implemented inhardware, software, firmware, or any combination thereof. If completelyor partially implemented in software, the software may be executed inone or more processors, such as a microprocessor, application specificintegrated circuit (ASIC), field programmable gate array (FPGA), ordigital signal processor (DSP). The software instructions may beinitially stored in a computer-readable medium and loaded and executedin the processor. In some cases, the software instructions may also besold in a computer program product, which includes the computer-readablemedium and packaging materials for the computer-readable medium. In somecases, the software instructions may be distributed via removablecomputer readable media, via a transmission path from computer readablemedia on another digital system, etc. Examples of computer-readablemedia include non-writable storage media such as read-only memorydevices, writable storage media such as disks, flash memory, memory, ora combination thereof.

Although method steps may be presented and described herein in asequential fashion, one or more of the steps shown in the figures anddescribed herein may be performed concurrently, may be combined, and/ormay be performed in a different order than the order shown in thefigures and/or described herein. Accordingly, embodiments should not beconsidered limited to the specific ordering of steps shown in thefigures and/or described herein.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope ofthe invention.

1. A method for luma-based chroma intra-prediction in a video encoder ora video decoder, the method comprising: down sampling a firstreconstructed luma block of a largest coding unit (LCU); computingparameters α and β of a linear model using immediate top neighboringreconstructed luma samples and left neighboring reconstructed lumasamples of the first reconstructed luma block and reconstructedneighboring chroma samples of a chroma block corresponding to the firstreconstructed luma block, wherein the linear model isPred_(C)[x,y]=α·Rec_(L)′[x,y]+β, wherein x and y are sample coordinates,Pred_(C) is predicted chroma samples, and Rec_(L)′ is samples of thedown sampled first reconstructed luma block, and wherein the immediatetop neighboring reconstructed luma samples are the only top neighboringreconstructed luma samples used; and computing samples of a firstpredicted chroma block from corresponding samples of the down sampledfirst reconstructed luma block using the linear model and theparameters.
 2. The method of claim 1, wherein the first reconstructedluma block is at a top border of the LCU.
 3. The method of claim 2,wherein the immediate top neighboring reconstructed luma samples arereceived from an top neighboring sample line buffer comprising a lastrow of reconstructed luma samples from each LCU of a row of LCUsimmediately above the LCU.
 4. The method of claim 1, wherein the firstreconstructed luma block is not at a top border of the LCU.
 5. Themethod of claim 1, wherein the left neighboring reconstructed lumasamples are immediately left of immediately left neighboringreconstructed luma samples of the first reconstructed luma block.
 6. Themethod of claim 2, further comprising: down sampling a secondreconstructed luma block of the LCU, wherein the second reconstructedluma block is not at the top border of the LCU; computing parameters αand β of the linear model using immediate top neighboring reconstructedluma samples, top neighboring reconstructed luma samples immediatelyabove the immediate top neighboring reconstructed luma samples, and leftneighboring reconstructed luma samples of the second reconstructed lumablock and reconstructed neighboring chroma samples of a chroma blockcorresponding to the second reconstructed luma block; and computingsamples of a second predicted chroma block from corresponding samples ofthe down sampled second reconstructed luma block using the linear modeland the parameters.
 7. A digital system configured to perform luma-basedchroma intra-prediction, the digital system comprising: means for downsampling a first reconstructed luma block of a largest coding unit(LCU); means for computing parameters α and β of a linear model usingimmediate top neighboring reconstructed luma samples and leftneighboring reconstructed luma samples of the first reconstructed lumablock and reconstructed neighboring chroma samples of a chroma blockcorresponding to the first reconstructed luma block, wherein the linearmodel is Pred_(C)[x,y]=α·Rec_(L)′[x,y]+β, wherein x and y are samplecoordinates, Pred_(C) is predicted chroma samples, and Rec_(L)′ issamples of the down sampled first reconstructed luma block, and whereinthe immediate top neighboring reconstructed luma samples are the onlytop neighboring reconstructed luma samples used; and means for computingsamples of a first predicted chroma block from corresponding samples ofthe down sampled first reconstructed luma block using the linear modeland the parameters.
 8. The digital system of claim 7, wherein the firstreconstructed luma block is at a top border of the LCU.
 9. The digitalsystem of claim 7, wherein the first reconstructed luma block is not ata top border of the LCU.
 10. The digital system of claim 7, wherein theleft neighboring reconstructed luma samples are immediately left ofimmediately left neighboring reconstructed luma samples of the firstreconstructed luma block.
 11. The digital system of claim 8, furthercomprising: means for down sampling a second reconstructed luma block ofthe LCU, wherein the second reconstructed luma block is not at the topborder of the LCU; means for computing parameters α and β of the linearmodel using immediate top neighboring reconstructed luma samples, topneighboring reconstructed luma samples immediately above the immediatetop neighboring reconstructed luma samples, and left neighboringreconstructed luma samples of the second reconstructed luma block andreconstructed neighboring chroma samples of a chroma block correspondingto the second reconstructed luma block; and means for computing samplesof a second predicted chroma block from corresponding samples of thedown sampled second reconstructed luma block using the linear model andthe parameters.
 12. The digital system of claim 8, further comprising:means for storing a last row of reconstructed luma samples from each LCUof a row of LCUs, wherein the means for computing parameters α and βreceives the immediate top neighboring reconstructed luma samples fromthe means for storing.
 13. A computer readable medium storing softwareinstructions that when executed in a digital system cause the digitalsystem to perform a method for luma-based chroma intra-prediction, themethod comprising: down sampling a first reconstructed luma block of alargest coding unit (LCU); computing parameters α and β of a linearmodel using immediate top neighboring reconstructed luma samples andleft neighboring reconstructed luma samples of the first reconstructedluma block and reconstructed neighboring chroma samples of a chromablock corresponding to the first reconstructed luma block, wherein thelinear model is Pred_(C)[x,y]=α·Rec_(L)′[x,y]+β, wherein x and y aresample coordinates, Pred_(C) is predicted chroma samples, and Rec_(L)′is samples of the down sampled first reconstructed luma block, andwherein the immediate top neighboring reconstructed luma samples are theonly top neighboring reconstructed luma samples used; and computingsamples of a first predicted chroma block from corresponding samples ofthe down sampled first reconstructed luma block using the linear modeland the parameters.
 14. The computer readable medium of claim 13,wherein the first reconstructed luma block is at a top border of theLCU.
 15. The computer readable medium of claim 14, wherein the immediatetop neighboring reconstructed luma samples are received from an topneighboring sample line buffer comprising a last row of reconstructedluma samples from each LCU of a row of LCUs immediately above the LCU.16. The computer readable medium of claim 13, wherein the firstreconstructed luma block is not at a top border of the LCU.
 17. Thecomputer readable medium of claim 13, wherein the left neighboringreconstructed luma samples are immediately left of immediately leftneighboring reconstructed luma samples of the first reconstructed lumablock.
 18. The computer readable medium of claim 14, the method furthercomprising: down sampling a second reconstructed luma block of the LCU,wherein the second reconstructed luma block is not at the top border ofthe LCU; computing parameters α and β of the linear model usingimmediate top neighboring reconstructed luma samples, top neighboringreconstructed luma samples immediately above the immediate topneighboring reconstructed luma samples, and left neighboringreconstructed luma samples of the second reconstructed luma block andreconstructed neighboring chroma samples of a chroma block correspondingto the second reconstructed luma block; and computing samples of asecond predicted chroma block from corresponding samples of the downsampled second reconstructed luma block using the linear model and theparameters.